1. Field of the Invention
This invention relates to ring laser gyroscopes and more particularly to controlling the optical path length in a laser gyro.
2. Description of the Prior Art
In operating a ring laser gyroscope, the optical path length of the laser beam travelling within an optical cavity formed by three or more mirrors must be adjusted to an integral number of wavelengths around the optical path. Temperature and other environmental variations can change the optical path length necessitating path length readjustment at frequent intervals. Usually path length is maintained at a resonant cavity peak where the output power, as evidenced by beam intensity, is held at near peak intensity by a control servo loop. Elements in this control loop include an optical sensor (e.g., photodiode), amplifier, phase detector, integrator and an electro-mechanical driver (a servo) which moves one of the mirrors toward or away from the center of the optical cavity plane along a normal to the mirror surface. To maintain the output beam intensity at near peak intensity, a sinusoidal "dither signal" is often applied to the servo drive, which tends to detune the cavity slightly. The sinusoidal signal is also applied as a reference signal to a phase detector to supply an average error signal back to the servo and bring the cavity tuning back to the peak resonant condition. Most ring gyros currently use this method of maintaining path length control.
As is well known to those skilled in the art, laser gyros typically employ some form of biasing to prevent frequency lockup at low rotation rates of the two optical beams counter-rotating in the cavity. In some laser gyros the biasing is mechanically induced by "dithering" the gyro (i.e., rotation of the gyro structure in the plane of the optical cavity). Other laser gyros employ a mirror having a magnetic coating to provide the bias needed to prevent frequency lockup of the counter-rotating beams at low rotation rates. Such "magnetic" mirrors can be switched to either of two stable states. The magnetic mirror provides a slight phase shift in the wavefront of an optical beam upon reflection from the magnetic mirror surface due to the "Kerr magneto-optic effect." For a given magnetic mirror state, this phase shift is different for the optical ight moving in one direction than it is for light moving in the opposite direction along the same optical path, resulting in a difference in frequencies between the two counter-rotating beams even when the gyro is not being rotated about its axis (normal to the laser plane). By using a combining prism, the two counter-rotating beams can be made nearly colinear to produce an output optical signal proportional to the difference in frequency between the two beams (because of the interference between the beam frequencies).
In maintaining the cavity tuning at a resonance peak, however, the magnetic mirror of a magnetically dithered gyro can introduce an additional error source over the more conventional "mechanically dithered ring laser gyro" or "rate biased ring laser gyro" when the optical path length is controlled by utilizing a dither signal to slightly detune the cavity as discussed above. As will be described more fully hereinbelow, dithering the path length control mirror position in a "magnetically dithered" gyro can introduce output rate errors, because the apparent output rate is altered by the dither signal.